I haven't found a good indicator myself. PEAR is installed per default as of PHP 4.3.0 - and you won't see a --with-pear switch in that version's phpinfo() output. Also, my hosting service is running PHP 4.2.3 and they have PEAR installed but again no indication of its presence in phpinfo() ...

PEAR is a collection of PHP packages (similar to Perl's CPAN, if you happen to know what that is). Those packages offer implementations of things you will typically need in a PHP application. PEAR is part of the standard PHP install as of PHP 4.3.0, but can easily be installed on older versions.

Starting with the next release, Geeklog will use the PEAR::Mail package to send all emails from within Geeklog - which means that emails can then be sent via sendmail and SMTP, too, and not only via PHP's built-in mail() function. We may use other PEAR packages in future versions. Also, Geeklog 2 will rely heavily on PEAR.